Their newborns also had a lower death rate: 0.2 percent died soon after birth, compared with 0.4 percent of babies born to unvaccinated moms.

It's not clear if the flu vaccine deserves the credit. But Dr. Jeanne S. Sheffield, the lead researcher on the work, said it's possible the vaccine helped by preventing severe cases of the flu.

"Can we say for sure that it's the vaccine? No," said Sheffield, a professor of obstetrics and gynecology at the University of Texas Southwestern Medical Center in Dallas.

But, she added, these findings suggest that the flu shot is at least safe, and possibly has a benefit against stillbirth.

Sheffield and her colleagues report the findings in the journal Obstetrics & Gynecology.

The Centers for Disease Control and Prevention (CDC) and other groups recommend that all pregnant women get a seasonal flu shot.

That's because pregnant women are more likely than other women their age to get a severe case of the flu or have complications, such as pneumonia. The flu is also thought to raise the risk of preterm delivery and fetal distress.

Still, Sheffield said, many women and doctors alike still have concerns about flu vaccine safety during pregnancy - especially the first trimester, which is when birth defects form and when most miscarriages happen.

But in this study, the researchers found that vaccination during the first trimester came with no increased risk of birth defects.

Despite recommendations to get the flu shot, most pregnant women do not. In the U.S., only between 10 percent and one-quarter of women have been vaccinated each flu season over the last couple decades, Sheffield's team notes.

Based on studies, that seems largely due to safety worries.

On the other hand, Sheffield said "it's amazing" how many women are unaware that the flu itself is considered a risk during pregnancy.

"The flu is a problem in pregnancy," she said. "But we have a vaccine to prevent it. And it's considered safe and effective in any trimester."

A CDC study published last year found "no unusual patterns" of pregnancy complications or newborn health problems among U.S. women who received the flu shot between 1990 and 2009.

Based on cases reported to the CDC's Vaccine Adverse Event Reporting System, the rate of miscarriage was 1.9 per one million vaccinated pregnant women.